Generally, in a color picture tube comprising an in-line type electron gun from which three electron-beams are emitted into a plane, as shown in FIG. 1, a center beam 1a and side beams 1b, 1c pass through main lenses 2a, 2b, 2c, respectively, to be focussed. In order that the side beams 1b, 1c are converged to a point 3 at the center of the phosphor screen together with the center beam 1a, a beam convergence angle .theta. is given between each of the side beams 1b, 1c and the center beam 1a. Also, a self-convergence deflection magnetic field is provided so that the convergence of the three beams 1a, 1b, 1c is performed automatically even at deflection to the peripheries of the screen. In the picture tube system thus constructed, the beam convergence angle .theta. affects the beam convergence characteristics over the entire phosphor screen.
The beam convergence angle .theta. can be given by arranging three electron guns obliquely. But, in that method, the beam convergence angle .theta. is liable to be varied by assembly errors that occur when the three independent electron guns are integrated into an assembled gun. Generally, therefore, a unitized electron gun structure in which the relative displacement of the three electron beams is expected to be small is employed as shown in FIG. 2. A unitized electron gun is described in detail in Japanese Patent Publication No. 4905/77 and others. The center axes 6b, 6c of the side beam apertures 5b, 5c among the beam apertures 5a, 5b, 5c at the end of a focussing electrode 4 on the side of an anode electrode, and the center axes 9b, 9c of the side beam apertures 8b, 8c among the beam apertures 8a, 8b, 8c at the end of the anode electrode 7 on the side of the focussing electrode are displaced or offset to each other to obtain axially asymmetric side main lenses 10b, 10c, so that the side beams are electrostatically deflected by the beam convergence angle .theta..
Incidentally, in this unitized electron gun structure, the beam convergence angle .theta. is determined by the relative positions of the side beam apertures 5b, 5c of the focussing electrode 4 and the side beams apertures 8b, 8c of the anode electrode 7, and therefore a very severe manufacturing accuracy is required for the electrodes 4 and 7.